1. Field of the Invention
The present invention relates to an ablative apparatus that can be used to treat atrial fibrillation and/or other cardiac arrhythmias by ablating cardiac tissue.
2. Description of the Related Art
Accounting for one-third of the hospitalizations for cardiac arrhythmia, atrial fibrillation (AF) is the most common arrhythmia (abnormal beating of the heart) encountered in clinical practice. (ACC/AHA/ESC Guidelines for the Management of Patients With Atrial Fibrillation) AF is a specific type of arrhythmia in which an abnormal beating of the heart originates in one of the heart's two atrium. Increasing in prevalence, an estimated 2.2 million Americans suffer from AF. (ACC/AHA/ESC Guidelines for the Management of Patients With Atrial Fibrillation) Underlying one out of every six strokes, AF doubles the rate of morbidity compared to patients with normal sinus rhythm. (ACC/AHA/ESC Guidelines for the Management of Patients With Atrial Fibrillation) Further increasing the clinical severity, the presence of AF leads to functional and structural changes in the atrial myocardium (cells responsible for the beating of the heart) that favors its maintenance. As such, AF is a serious disorder requiring medical intervention.
Administering drugs that alter the electrical properties of atrial myocardium has been effective in treating less severe cases of AF. (Hurst's the heart, page 836) However, such drugs often lead to the creation of pro-arrhythmic conditions thereby resulting in the treatment of one type of arrhythmia only to create another. Due to the increased risk of stroke, it is advised that all patients with AF, despite the successfulness of drug therapy, be prescribed warfarin or other anticoagulants to inhibit the formation of blood clots. (Hurst's the heart, page 833) Besides being difficult to dose, warfarin has several complications associated with its long term use. Altering the metabolism of other drugs, warfarin is known to induce several adverse interactions with other medications commonly prescribed to elderly patients, who are at increased risk of developing AF.
AF originates in regions of myocardium contracting, or beating, out of step with the rest of the heart. Heart cells contract in response to electrical stimulation. In a healthy heart, the electrical stimulation signaling contraction originates from the sinus node (the heart's natural pace maker) and spreads in an organized manner across the heart. In a heart plagued with AF, a region of myocardium elicits a mistimed contraction, or heart beat, on its own or in response to an electrical signal generated from somewhere other than the sinus node. Generating an electrical signal, the mistimed contraction spreads across the heart, inducing contractions in neighboring regions of the heart. Inducing the formation of scar tissue on the heart by ablating, cutting, or otherwise injuring tissue in regions in which AF originates has been shown to be affective in treating AF. The logic behind this treatment is to terminate AF by removing the heart cells responsible for its presence, while preserving healthy cells. Creating scar tissue barriers as to prevent the spread of electrical signals from mistimed contractions has also been shown to be effective in treating AF. (Hurst's the heart, page 838)
Successful surgical intervention eliminates the need for continued warfarin treatment in most patients. (Hurst's the heart, page 839) Initially surgical treatment was reserved for patients undergoing additional cardiac surgery, such as valve repair or replacement. (Hurst's the heart, page 838) The high success rate and efficacy of surgical intervention in the treatment of AF has spurred the development of cardiac catheters capable of therapeutically ablating cardiac tissue without the need for open chest or open heart surgery.
Heart surgery preformed by means of catheter involves, in it basic conception, the insertion of a catheter either into a patient's vein or chest cavity. The catheter is then advanced to the heart. When the catheter is inserted into a patient's vein, the catheter is advanced into one of the heart's four chambers. When the catheter in inserted into a patient's chest, the catheter is advanced to the outer walls of the patient's heart. After the catheter reaches the patient's heart the surgeon utilizes the catheter to ablate, damage or, kill cardiac tissue. The ideal catheter induced lesion is one that is created from the epicardium (outside) of the beating heart, is able to go through epicardial fat, is performed rapidly over variable lengths, is transmural, causes no collateral injury, and can be applied at any desired anatomic location. (Williams et al., 2004) Ablating cardiac tissue by heating the tissue to 50 degrees Celsius has become the preferred means of inducing lesions (Williams et al., 2004). Cardiac catheters employing a variety of thermal ablative energy sources have been developed, none of which are capable of inducing an ideal lesion.
Catheters utilizing radio frequency as an ablative energy source, the current gold standard, are incapable of creating an ideal lesion. (Cummings et al., 2005) In particular, radio frequency catheters have a difficult time creating ablations through the epicardial fat surrounding the heart. Furthermore, inducing deep lesions with radio frequency is not possible without inflicting collateral damage from surface burning and steam popping. (Cummings et al., 2005) Steam popping is the phenomenon in which cells become heated to such a point their internal fluids begin to boil, producing steam that bursts the cell. Simultaneously cooling the site of radio frequency administration reduces the incidence of surface burns but does not reduce the risk of steam popping. (Cummings et al., 2005) In an effort to overcome the shortcomings of radio frequency induced lesions, catheters employing novel energy sources have been developed.
In hopes that microwaves would provide sufficiently deep lesions, catheters employing microwaves as an ablative energy source have been developed. Because the penetration of microwaves into tissue has a steep exponential decline, it has been found necessary to bring the catheter into close contact with the tissue in order to induce deep lesions. (Cummings et al., 2005) Furthermore, fat continues to be a significant barrier. (Williams et al., 2004)
Lasers have also been applied as an ablative energy source within catheters. Although high powered lasers carry a high risk of crater formation at the site of application, low energy lasers produce lesions with a depth related to the duration of application. (Cummings et al., 2005)
Capable of penetrating fat and inducing fasts lesion at specific depths when focused, high intensity ultrasound has been predicted to be an advantageous source of ablative energy in catheters. (Williams et al., 2004)
An alternative to ablation by heating is the practice of ablating tissue by freezing. Severe cold, also know cryogenic energy, as an ablative energy source has the advantages of avoiding clot formation. (Williams et al., 2004) Another advantage of catheters employing cryogenic energy is the ability to temporary paralyze regions of myocardium tissue as to test the benefit of a planned lesion. When a region of tissue is paralyzed by freezing it can no longer initiate an arrhythmia. If paralyzing a region of the heart completely or partial restores a normal heart beat, the surgeon knows she has her catheter aimed at the right spot.